In tunnels and mines, the roof and walls of the excavation must be supported during the construction phase with anchors to reinforce the rock and keep it from falling. Rock anchors are installed by drilling holes in the rock and then inserting and fastening the rock anchor. The method of installation depends on the type of anchor. U.S. Pat. Nos. 4,798,501; 4,984,937; 5,531,545; 5,586,839; and 5,954,455 and United States Patent Publication 2009/0191007 A1 disclose various types of rock anchors.
Mechanical anchors are made from solid steel bars with threaded ends. One end has a nut thereon to become the bolt head and the other end (toe) receives an expansion shell (to fasten the toe upon spinning the bolt). Mechanical anchors are easily installed in the field since the installation procedure requires conventional drills, such as jackleg or stoper, to spin the nut on the bolt head once it is inserted in the drill hole. However, mechanical rock anchors are not as strong as cable bolts.
Cable bolts are made of multi-strand cables from high strength steel. Fastening the cable bolt in the bore hole is usually done by pushing the cable bolt behind one or more resin grout cartridges in the bore hole. By pushing and spinning the cable bolt into the bore hole, the resin grout cartridges rupture and the resin mixes and hardens. Spinning the cable is done with special mechanical devices designed to grip firmly on the portion of the cable bolt outside the bore hole and to spin it while pushing it. In order to secure the cable bolt head, the current practice of cable bolting as rock support system for tunnels and mines requires the use of a plate that is retained against the rock surface by a wedge and a barrel mounted on the cable bolt portion outside the bore hole to secure the bolt head at the rock surface.
One drawback of such cable bolts is that their installation is tedious and time consuming. It is a two-step procedure as it requires special jacking equipment to be brought in to push in the wedge inside the barrel. Another drawback is that it is cumbersome to apply tensioning to the cable bolt head with this method. Typically, tensioning is limited to a few metric tons (3 to 4) with this method. Furthermore, it is difficult to spin the cable bolt for resin mixing applications without the help of yet another special device in the form of a metal tube that fits the stranded body of the cable bolt portion outside the bore hole to force it to spin. This further increases the installation time and makes it more cumbersome.
Therefore, there is a need for a cable bolt that can easily be installed and tensioned.